The invention relates to a colour display tube comprising:
an envelope with a longitudinal axis, having a neck portion, a funnel portion and a window portion; PA1 an electron gun arranged in the neck portion; PA1 a display screen having a short axis and a long axis and a pattern of phosphor lines parallel to an axis of the display screen on the inner surface of the window portion; PA1 a colour selection means arranged proximate to the display screen; PA1 a magnetic shielding structure arranged within the funnel-shaped portion, which shielding structure has two long wall portions parallel to the long axis of the display screen and two short wall portions parallel to the short axis of the display screen, and an aperture at its gun-sided end, which aperture extends transversely to the longitudinal axis and constitutes a scanning aperture for electron beams produced by the gun and scanning the display screen. PA1 1. Welding on the strips is a relatively expensive operation, PA1 2. The spot welds are not very reliable (loosening), PA1 3. Oil and grease residues behind the welded strips are difficult to remove (cathode poisoning). PA1 local deformation (by means of a centre punch or a laser beam) in the area between the aperture and the edge; PA1 local diffusion of a non-magnetic material, such as aluminium suitable for use in an evacuated space, in the area between the aperture and the edge.
A colour selection means is herein understood to mean, for example, an apertured shadow mask sheet or a wire mask.
The ratio between the dimension of the long central axis and the dimension of the short central axis of the display screen characterizes the picture format.
In a (colour) display tube the earth's magnetic field deflects the electron paths, which without any measures may be so large that the electrons impinge upon the wrong phosphor line (mislanding) and produce a discolouration of the picture.
Modem display tubes are provided with an internal magnetic shielding structure (shield) to limit the deviation of the electron path due to the earth's magnetic field. A complete shielding is not possible due to an aperture which is required for passing the electron beam. A horizontally directed spot displacement caused by the lateral earth's magnetic produces a risk of discolouration (N effect) in the corners only. The internal residual field can be influenced by means of an additional measure in such a way that the electron beam still passes the mask at the desired angle. This measure involves, for example the use of a shield with "vertically" directed slits (situated in a plane parallel to the short axis of the display screen). The internal residual field is then influenced in such a way that there is less spot displacement in the horizontal direction. The slits enhance the magnetic resistance in the shield material in the horizontal direction so that there is more spot displacement in the vertical direction. However, for picture tubes with phosphor lines extending in this direction this is no problem because it does not lead to discolouration. In the extreme case the shield is split fully magnetically ("split shield"). Overcompensation of the N effect may then even occur.
A problem of "vertically" directed slits is that the slit length is to be limited to ensure the mechanical stability of the shield so that an unacceptable spot displacement remains in the corners, particularly in large tubes. In large tubes having a picture diagonal of 41 cm or more, such as 80 FS ("Flat Square") and 36 inch WS ("Wide Screen") it has been attempted to lengthen the slits to a maximum extent and to restore the resultant loss of mechanical strength by welding on supporting strips of non-ferromagnetic material. However, the following problems then occur.